In a liquid crystal device, if impurities responsive to an electric field (hereinafter referred to as "field responsive impurities") are mixed in a liquid crystal, the device performance, such as a response speed and contrast, deteriorates and the service life of the device shortens. The field responsive impurities are defined as chemical species capable of moving or transporting electric charges within the device upon application of the electric field. The field responsive impurities include protons, organic ions, inorganic ions, compounds having a hydrogen bonding ability, compounds having an electron transporting ability, compounds having a large dipole moment, compounds having a large polarizability, and the like. It is therefore indispensable to improve the manufacturing process in order to prevent contamination of the device with the field responsive impurities by detecting, identifying, and determining them mixed in the device. At this time, to properly determine which one of processes should be modified and how to modify it, the identification of the mixed impurities will be important.
To evaluate impurities, a method of measuring a voltage retention ratio of the liquid crystal device at high temperature has been conventionally employed. This method enables to evaluate the liquid crystal device in the final state of the construction thereof. However, this method requires much time and labor. In addition, it is difficult to specify causative materials for impurities and a process in which impurities are mixed in the liquid crystal device.
It is an object of the present invention to provide a liquid crystal evaluation method capable of identifying field responsive impurities contained in a liquid crystal device simply and with high sensitivity, and an apparatus for realizing the evaluation method.